1. Field of the Invention
This invention relates to a nonflammable paper, particularly a nonflammable paper having a low basis weight.
2. Description of the Prior Art
(1) Nonflammable paper and fire-retardant paper so far used include an asbestos paper composed mainly of asbestos and a paper obtained by subjecting usual pulp as raw material to paper-making, followed by treatment with various flame-retardants. The former has superior properties such as heat resistance, chemical resistance, etc., but on the other hand, since asbestos has been regarded as a substance causing lung cancer, a severe regulation has come to be carried out.
On the other hand, in the case of the latter, since it generates a poisonous gas or fuming at the time of its ignition, a serious problem has been raised in the aspect of safety.
(2) In recent years, nonflammable papers using a material of non-pollution-nuisance have come to be desired, and in accordance with this desire, a nonflammable paper composed mainly of aluminum hydroxide has been developed.
The aluminum hydroxide paper has non-combustibility, non-pollution-nuisance, self-extinguishing properties, high brightness, etc., whereas in the case of a low basis weight, particularly in the case of 120 g/m.sup.2 or less, it has had drawbacks that the yield at the time of paper-making lowers and also the strength and non-combustibility lower to a large extent to make it impossible to obtain a product which is endurable to use as a nonflammable paper.
(3) Recently, a nonflammable paper composed mainly of calcium silicate has been developed (Japanese patent application laid-open No. Sho 58-98495/1983), but since calcium silicate hydrate has a high dehydration temperature, there is a drawback that it is inferior in non-combustability. Thus, in order to obtain a sufficient non-combustibility (flameproofness, first grade), it is necessary to make the mixing proportion of pulp 5% or less. As a result, when the basis weight is made 100 g/m.sup.2, the tensile strength becomes as notably low as about 0.5 Kg/15 mm, that is, does not reach the practical strength of paper.